The present invention relates to devices for controlling the level of a liquid in a container. More particularly the invention relates to a device for detecting the water level inside a boiler of an espresso coffee machine, said device using a capacitive sensor associated with means for detecting changes in the sensor capacitance of said capacitive sensor and for automatically controlling water flow into the boiler, so as to keep the desired level constant.
As known, espresso coffee machines comprise a boiler for containing and heating the water, a water pressurising device, one or more coffee delivery groups and other internal and external accessories. The whole of those units is enclosed within an external casing.
Generally the boiler is coupled with a duct for cold water supply, by which the optimum water level is recovered during use, preferably in automatic manner. An electrically actuated valve is therefore present along the supply duct, which valve is controlled by a suitable system monitoring the level in the boiler and actuating the electric valve in correspondence with a minimum level and until a desired level is reached.
Many detectors used for monitoring the level comprise a probe equipped with one or more rods of electrically conductive material, which are introduced directly in the boiler or in a suitable vessel in communication with the boiler. When contacting the liquid, said rods signal the level reached by the liquid to the control system. Indeed such probes exploit the conductive properties of the liquids and allow measuring the electrical resistance between each rod and a ground contact in the boiler. Actually the boiler is generally made of metal, in order to bear the high temperature and pressure to which it is exposed.
FIG. 1 shows an example of a prior art level probe employed in an espresso coffee machine. Probe 2 comprises a glass tube 4, located between a bottom coupling member 6 and a top coupling member 8, and a central electrically conductive rod 10, located inside tube 4. The two coupling members 6, 8 establish communication between the interior of tube 4 and the interior of a boiler, the bottom and the top of the boiler, respectively. Thus the liquid contained in the boiler enters tube 4 and keeps therein a level corresponding to the level in the boiler. Of course, for a correct operation, the probe and the boiler must be located at about the same level. If the liquid level is above dashed line 14, rod 10 is in contact with the liquid and its electrical resistance, measured relative to the boiler ground, is very low. When on the contrary the liquid level decreases below dashed line 14, the measured resistance is higher, since air or steam in the upper boiler part have greater resistance than water. The control system exploits this principle to actuate the electric valve controlling water flow into the boiler.
However, direct contact of rod 14 with the liquid contained in the boiler may cause some drawbacks, especially as time goes by. The particular environment in which the rod is immersed, namely hot water or steam, promotes metal oxidation and causes the formation of calcareous residuals and the deposition of foreign substances possibly present in water. All those phenomena, which as time goes by make the probe resistance vary, can result in malfunctioning of the control system and can give rise to critical situations, even from the safety standpoint, in the overall machine operation.
GB-A 696 907 discloses a level regulating device employing a capacitive sensor coupled to a glass tube communicating with the container where the level is to be regulated. The sensor includes a capacitors of which a first electrode comprises a pair of metal sleeves surrounding the glass tube and the other electrode is a wire immersed in the liquid. The device has the same drawbacks as the prior art discussed above. Moreover, a two-piece electrode is required to detect a maximum and a minimum admitted level.
EP-A 0 377 508 discloses a capacitive liquid level sensor, in particular to detect the lower limit of acceptable fuel level within the fuel reservoir in a motor vehicle. That sensor includes a capacitor, of which one armature is a plate directly mounted on a reservoir wall, at a position to sense the lower level limit, and the second armature is the reservoir ground. The device is scarcely flexible, since the sensor is to be directly mounted on the wall of the reservoir. No provision is made for a direct inspection of the level.
It is an object of the present invention to provide a device which is of simple construction and is capable of detecting the liquid level in precise manner, while ensuring high safety in all operating conditions.
The above and other objects are achieved by the device for controlling the level in a boiler of a coffee machine made in accordance with the invention, as claimed in the appended claims.
Advantageously the device made in accordance with the invention can be employed in professional or super-automatic espresso coffee machines, i.e. machines equipped with a system for the automatic filling of the hot water boiler.